Supermarket chain Morrisons has reported brisk sales of potatoes and carrots from a trial of sales of so-called “wonky” seasonal vegetables in stores in Yorkshire and the north-east, and has also relaxed its specifications for brussels sprouts to allow fewer to be wasted. An earlier attempt to sell wonky courgettes was branded “pathetic” by chef and campaigner Hugh Fearnley-Whittingstall.

The imperfect vegetables – potatoes, carrots, onions and parsnips – in Morrisons’ latest initiative are all products that are popular at this time of year as staple items on the Christmas dinner plate. They have been selected from farmers’ crops because they are either misshapen, have growth cracks, or are much smaller or larger than average. They have been discounted by about a third in price to reflect that customers might need to spend more time peeling them or might not be able to use the whole vegetable.

The company said it had chosen Yorkshire and the north-east for the trial because it has a large number of stores in those regions and can monitor them more easily from its head office in Bradford.

Ahead of the peak brussels sprout-buying week of the year, Morrisons has also relaxed its specifications to allow so-called “monster” sprouts – up to a third bigger than their usual size because of the unusually warm weather.

Morrisons’ average weekly supply of 3,700 tonnes of potatoes soars to 6,500 tonnes during Christmas week, while for sprouts it leaps from 180 tonnes to 650 tonnes. And 20% of sprouts sold during the year are tend to go in a four- to six-day window in the runup to Christmas.

The trial in 75 branches started earlier in December and will be rolled out to a further 75 stores, including outlets in the Midlands, and to online delivery in early January. Morrisons claimed initial sales had been “promising”, with sales close to expectations on potatoes, carrots and onions, although misshapen parsnips were less popular. Sales in the first week have amounted to approximately £25,000.

But Drew Kirk, Morrisons’ produce director, admitted that shoppers often sought perfection for their Christmas feast. “Customers often want their festive meals to look and taste perfect, so it is going to be very interesting to see whether they take to these wonky vegetables over the Christmas period.

“Sales of our wonky vegetables have been encouraging, as they have been priced at a discount and sold next to the class one crop to give customers the choice. If the sales tell us that customers really like these products, we will roll them out to other parts of the UK.”

Morrisons’ competitor Asda now sells wonky vegetables and fruit as standard after research for its own trial showed that 65% of its customers were open to the idea of buying oddly shaped fresh produce, while 75% would definitely “buy wonky” if it was cheaper. Sainsbury’s has also relaxed its rules on the cosmetic appearance of fresh produce, and Tesco has experimented with putting misshapen fruit and vegetables on offer to encourage people to buy them.

Morrisons is stepping up its drive to cut food waste throughout its supply chain. Crucially, it is doing so after pledging in the BBC1 programme Hugh’s War on Waste that it would look at options to sell more imperfect vegetables. In an embarrassing development for Morrisons, the chef who presented the show, Hugh Fearnley-Whittingstall, was filmed giving away oversized and curvy parsnips outside a branch in Wimbledon, to highlight the food waste he says is due to supermarkets’ excessively exacting cosmetic standards. In response, Morrisons undertook a trial of selling wonky courgettes alongside “class one” courgettes. The “ugly” ones, however, sold much more slowly.

Fearnley-Whittingstall told the Guardian in an interview in November: “When you see the frankly pathetic little trial that Morrisons did with those courgettes, where they put some really substandard, squashy-ended ones in one pile next to some gleaming, perfect ones at the same price, would you believe people went for the really lovely ones? That’s not what we’re asking supermarkets to do.”

The timing of Christmas Day on Friday means that supermarkets are preparing for “trolley gridlock” as shoppers stock up for their festive food shop from this weekend onwards, with next Wednesday set to be the busiest of the year. Global information company Nielsen said shoppers would spend more than £6.4bn at leading supermarkets in the last two weeks in the approach to Christmas, with 60% of consumers delaying their shopping until the last few days.

“Shoppers will leave it late this year,” said Mike Watkins, Nielsen’s UK head of retailer and business insight. “This could provide a welcome boost for grocery retailers, particularly with an extra weekday of trading compared with last year. But the discounters could benefit – a third of people are expecting to do their main Christmas shop at Aldi or Lidl.”